Method of removing entrapped gas and/or residual water from glass

ABSTRACT

A method of forming an article, such as a low loss optical waveguide, by applying to a starting member a layer of glass soot to form a porous body. The porous body is then placed in a controlled environment in which a predetermined desired concentration of gases is maintained. The porous body is heated below the sintering temperature of the glass to permit entrapped gas to escape therefrom and the temperature is maintained until an equilibrium is reached between the partial pressure of the entrapped gas in the porous body and the partial pressure of the same gas in said environment. Thereafter, the porous body is further heated to at least the sintering temperature of the glass to sinter the soot particles and to form a consolidated dense member which may thereafter be formed into a desired shape while within said environment.

on 308681170 J, Nd 7 o a (9 Umted State g tg 1111 3,868,170 DeLuca 145118611.25, 1975 METHOD OF REMOVING ENTRAPPED GAS Primary Examiner-S. LeonBashore Assistant Examiner-Frank W. Miga AND/OR RESIDUAL WATER FROMGLASS [75] Inventor: Robert D. DeLuca, Big Flats, N.Y. Attorney Agent,or Firm walter S. Zebrowski; [73] Assignee: Corning Glass Works,Corning, Clarence R. Patty, Jr.

[22] Filed: Mar. 30, 1972 [57] ABSTRACT [21] App]. No.: 239,746 A methodof forming an article, such as a low loss optical waveguide, by applyingto a starting member a 52 us. c1. 350/96 wo, 65/3, 65/18, 9 glass "9" a65/30 65/32 65/60 161/1 body 15 then placed 1n a controlled env1ronment1n which a predetermined desired concentration of gases [5!] Int. ClG02g 5/14, C03c 15/00 1s mamtamed. The porous body 1s heated below the[58] Fleld of Search 65/32, 30, 3, 134, 18,

65/60 350/96 WG' 161/1 smtermg temperature of the glass to permltentrapped gas to escape therefrom and the temperature is main- 56 Rf Ctd tained until an equilibrium is reached between the l e erences partialpressure of the entrapped gas in the porous UNITED STATES PATENTS bodyand the partial pressure of the same gas in said 3,228,760 H1966 Jacketal ..65/32 environment. Thereafter, the porous body is further 31281761V1966 Jack etal heated to at least the sintering temperature of the313384394 Davy glass to sinter the soot particles and to form a consoli-3527 7]] 9/1970 B rber et a] 3'647'406 3H9," masher dated dense memberwh1ch may thereafter be formed 3:659:9l5 5/1975 Murcia 51:11:: 65/30 x adesired Shape whi'e w'thin Said envimnmem' 3,71 L262 l/l973 Keck et al l3.756.798 9/1973 Ernsberger 24 C 10 Drawing F s 'Q i 7 f) PAIEHTEU3.868.170

sum 2 9f 1 A F keel/mole 1 l I I I I I 800 I000 I200 I400 I600 I800 2000TEMPERATURE C FREE ENERGY CHANGES FOR Ti0 OXIDATION-REDUCTION REACTIONS.

Fig. 2

PAIEIIIEU FEB25I9Y5 sum 3 0 I I l I I I I I I I I I I l I I I I I I 800I000 I200 I400 I600 I800 2000 GLASS TEMPERATURE C EFFECT OF TEMPERATUREAND OXYGEN CONCENTRATION 0N Ti IN 2.5 WEIGHT PERCENT Ti0 AND 97.5 WEIGHTPER CENT Si0 GLASS.

F fg. 3

PATH-HEB FEMEIWS 3868.170

' sum u o T l l l l l l 800 I000 I200 I400 I600 I800 2000 GLASSTEMPERATURE C EFFECT OF H2,OH ON n IN 2.5 WEIGHT PERCENT Ti0 AND 97.5WEIGHT PERCENT SiO GLASS .QEEHEEU 3.868.170

sum 5 BF 9 I l 1 I 800 I000 I200 I400 I600 I800 2000 GLASS TEMPERATURE CEFFECT OF H2, H2O ON n IN 2.5 WEIGHT PERCENT TiO AND 97.5 WEIGHT PERCENTSiO GLASS Fig. 5

PATENTEU 3,868,170

mm a or f -&

x 2 a z a \4 28 Q 24-":

Fig. 7

METHOD OF REMOVING ENTRAPPED GAS AND/OR RESIDUAL WATER FROM GLASSBACKGROUND OF THE INVENTION I. Field of the lnvention This inventiondeals with a method of forming high optical purity blanks that are freeof residual water and entrapped gases from which high optical qualitylenses, prisms, optical waveguides, and the like can be made.

Waveguides used in optical communications systems are referred to as"optical waveguides" and are normally constructed from transparentdielectric material such as glass or plastic.

It is well known to one skilled in the art that light can be caused topropagate along a transparent fiber structure which has a higherrefractive index than its surroundings. The ordinary use of such opticalwaveguides is to transmit a signal or an image, that is light which hasbeen modulated in some form, from one point to another. To be effective,optical fibers produced for these purposes must avoid excessiveattenuation of the transmitted light usually resulting from one or morecauses such as scattering, absorption, or the like. One serious cause oflight attenuation by scattering or absorption is due to the presence ofresidual water or other entrapped gases within the glass material.Further, to be an effective transmitting media for an opticalcommunication system, an optical waveguide should not only transmitlight without excessive attenuation, but should allow only preselectedmodes of light to propagate along the fiber.

Operational theories and other pertinent information concerning opticalwaveguides may be found in US. Pat. No. 3,157,726 issued to Hicks etal.; in the publication entitled Cylindrical Dielectric Waveguide Mode"by E. Snitzcr, Journal of the Optical Society of America. Vol. 5 l No.5. pages 49l498, May 196] and in "Fiber Optics Principles andApplilcations by N. S. Kapany, Academic Press (l967).

The propagatioin of light waves is governed by the same laws of physicsthat govern microwave propagation, and therefore, can also be studied interms of modes. Since each mode of light traveling around a glass fiberstructure propagates at its own inherent velocity, it can be shown thatinformation initially supplied to all modes will be dispersed aftertraveling a given length offiber due to different propagationvelocities. lf light propagation along an optical fiber could berestricted to preselected modes, clearly more effective informationtransmission would result. Producing a satisfactory optical waveguidehas been one of the more difficult problems in the development of aneffective optical communication system.

ll. Description of the Prior Art A method heretofore used for producingan optical fiber is described as follows. A rod of glass possessing thedesired core characteristics was inserted into a tube of glasspossessing the desired cladding characteristics. The temperature of thiscombination was then raised until the viscosity of the materials was lowenough for drawing. The combination was then drawn until the tubecollapsed around and fused to the inside rod. The resulting laminatedrod was then further drawn until it cross-sectional area was decreasedto desired dimensions. During the drawing process, the rod and tubewould normally be fed at different speeds to attempt to produce a fiberwith the desired core to cladding diameter ratio. This method, however,has been sometimes unsatisfactory because of the particular difficultyin maintaining the core and cladding dimensions. Further, residual waterin ordinarily produced glass causes absorptive attenuation. By residualwater in glass is meant that the glass contains a high level ofOH, H andH 0. One explanation of residual water may be found in US Pat. No.3,531,271 to W. H. Dumbaugh, Jr. An aditional problem is that numeroustiny air bubbles and foreign particles are often entrapped within theglass material or at the core and cladding interface and become a sourceof light scattering centers. ln addition, the core and claddingmaterials of any waveguide must be selected so that there is a precisedifference between the two indices of refraction. Glass tubes and glassrods which simultaneously have precise differences in their indices ofrefraction, low residual water content, similar coefficients ofexpansion and similar viscosities are not readily available. Variationsin core diameter or in either index of refraction may significantlyaffect the transmission characteristics of a waveguide.

SUMMARY OF THE INVENTION It has been found that, in order to producesuitable optical waveguides, attenuation must be kept to below about 20db/km over the entire wavelength range, for example, about 600-1 ,500nm. In order to achieve this level of light attenuation in the longlengths of optical fibers which are used in present and proposed opticalcommunication systems, it has been found that among other requirementsresidual water within the glass must be reduced to a level of less thanabout 20 parts per million (ppm).

As used herein generally, the term entrapped gas shall be understood toinclude residual water.

It is an object of the present invention to provide an article andmethod for producing it which overcome the heretofore noteddisadvantage.

Other objects of the present invention are to provide a method ofproducing an article which is substantially free of residual water andother entrapped gases, from which article may be formed an opticalwaveguide that will not cause excessive light absorption losses, thatwill not cause excessive dispersion of the transmitted light, that doesnot have light scattering centers within the glass material and at thecore and cladding interface, and that has otherwise improvedcharacteristics.

Broadly, according to this invention an article formed of glasssubstantially free of residual water and other entrapped gases isproduced by providing a suitable starting member and then depositing aquantity of glass soot by flame hydrolysis on the starting member toform a porous body. The porous body is then placed in a controlledenvironment within which a predetermined desired concentration of gasesis maintained. The porous body is then heated to a temperature belowthesintering temperature of the glass to permit entrapped gas to escapetherefrom. The temperature is maintained until an equilibrium is reachedbetween the partial pressure of the entrapped gas in the body and thepartial pressure of the same gas in the environment. The body is thenheated to at least the sintering temperature of the glass to sinter thesoot particles and form a consolidated dense member. If desired, thestarting member may be removed from said consolidated memeber outside ofsaid environment. Further, if desired. the consolidated dense member maythereafter be formed to a desired shape within the same environment.

If an optical waveguide is to be formed from the consolidated densemember, a coating of glass having an index of refraction lower than thatof the consolidated member may then be applied to the exterior surfaceof the member by any desired means such. for example. as flamehydrolysis. If necessary, the coating is sintered and consolidated asabove described. Such a coating will form the waveguide cladding whilethe consolidated member will form the waveguide core. With the startingmember removed, a substantially cylindrical hollow assembly is formed.This assembly is then heated to a temperature at which the materialshave a low enough viscosity for drawing and is drawn to reduce thediameter thereof until the hole resulting from the removal of thestarting member is collapsed. That is, the longitudinal hole is sealedto form a solid rod surrounded by the cladding coating of glass.Thereafter, continued drawing of the composite structure further reducesthe diameter thereof to form a glass optical fiber which possesses thecharacteristics of the desired optical waveguide. That is, it transmitspreselected modes of light without excessive attenuation or absorptionlosses. does not cause excessive dispersion of the transmitted light,and provides an improved claddingcore interface. The waveguide shouldpreferably be drawn in the same controlled environment. The glass of theoptical waveguide so formed contains less than 20 ppm of residual water.

These and additional objects, features and advantages of the presentinvention will become apparent to those skilled in the art from thefollowilng detailed description and the attached drawing, on which, byway of example, only the preferred embodiments of this invention areillustrated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration ofa means ofapplying a coating of glass soot to the outside of the starting member.

FIG. 2 is a graph illustrating free energy changes for TiO-oxidation-reduction reactions.

FIG. 3 is a graph illustrating the effect of temperature and oxygenconcentration on reduction of TiO to form Ti in a 2.5 percent by weightTiO and 97.5 percent by weight Sio glass.

FIG. 4 is a graph illustrating the effect of H OH on Ti in a 2.5 percentby weight TiO and 97.5 percent by weight SiO glass.

FIG. 5 is a graph illustrating the effect of H H O on Ti in a 2.5percent by weight TiO and 97.5 percent by weight SiO glass.

FIG. 6 is an illustration of a means of removing entrapped gas from aporous body.

FIG. 7 is an illustration of a .iieans of sintering a porous body toform a dense member.

FIG. 8 is a fragmentary cross sectional elevation illustrating a meansof removing the starting member.

FIG. 9 is an illustration of a means of applying a coating to theoutside surface of the consolidated dense member of the presentinvention.

FIG. 10 is a fragmentary elevation partly in cross section of an opticalfiber being formed.

DI'IIAILIiI) DESCRIPTION ()I THE INVENTION It is to be noted that thedrawings are illustrative and symbolic of the present invention andthere is no intention to indicate the scale or relative proportions ofthe elements shown therein. For the purposes of simplicity, the presentinvention will be substantially described in connection with theformation of a low loss optical waveguide although this invention is notintended to be limited thereto.

Referring to FIG. I. a layer 10 of glass is applied to a substantiallycylindrical starting member or rod l2 by means of flame hydrolysisburner I4. Fuel gas and oxygen or air are supplied to burner 14 from asource not shown. This mixture is burned to produce flame 16 which isemitted from the burner. A gas-vapor mixture is hydrolyzed within flame16 to form a glass soot that leaves flame 16 in a stream 18 which isdirected toward starting member 12. The flame hydrolysis method offorming layer 10 is hereinafter described in detail. Starting member 12is supported by means of support portion 20, and is rotated andtranslated as indicated by the arrows adjacent thereto in Flg. I foruniform deposition of the soot. It is to be understood that an elongatedribbon burner, not shown, that provides a long stream of soot could beused in place of the substantially concentric burner illustrated in FIG.I whereby starting member 12 would only have to be rotated. Further, aplurality of burners 14 could be employed in a row to similarly requireonly rotation.

Since glass starting member 12 is ultimately removed, the material ofmember 12 need only be such as to have a composition and coefficient ofexpansion compatible with the material of layer 10. A suitable materialmay be a normally produced glass having a composition similar to that ofthe layer 10 material although it does not need the high purity thereof.It may be normally produced glass having ordinary or even an excessivelevel of impurity or entrapped gas that would otherwise render itunsuitable for effective light propagation. The starting member may alsobe formed of graphite or the like.

In the manufacture ofoptical waveguides, the materials ofthe core andcladding of the waveguide should be produced from a glass having minimumlight absorption characteristics, and although any optical quality glassmay be used, a particularly suitable glass from which to make an opticalwaveguide is fused silica. For structural and other practicalconsiderations, it is desirable for the core and cladding glasses tohave similar physical characteristics. Since the core glass must have ahigher index of refraction than the cladding for proper operation, thecore glass may desirably be formed of the same type of glass used forthe cladding and doped with a small amount of some other material toslightly increase the index of refraction thereof. Therefore, if purefused silica is used as the cladding glass, fused silica doped with thematerial to increase the index of refraction can be used as a coreglass.

There are many suitable materials that can satisfactorily be used as adopant alone or in combination with each other. These include, but arenot limited to, titanium oxide, tantalum oxide, tin oxide, niobiuimoxide, zirconium oxide, aluminum oxide, lanthanum oxide, and germaniumoxide. The amount of dopant used should be kept to a minimum for variousreasons. First, since additional doping material would cause the indexof refraction to increase, the difference between the index ofrefraction of the cladding glass'and the core glass will also increaserequiring a decrease in the allowable core diameter of the waveguide toobtain a waveguide having the same operating characteristics. Second, ifan excessive amount of doping material is added to the base material, aloss of light transmission will result. Desirably, as small yet preciseamount of dopant should be added to the base material for the primarypurpose of changing the index of refraction. For the purposes of forminga waveguide in accordance with the present invention, the amount ofdopant is preferably maintained below about percent by weight of thetotal composition.

A particularly effective method of forming or applying a layer orcoating is accomplished by a flame hydrolysis process similar to thatdescribed in US. Pat. No. 2272342 issued to J. F. Hyde or U.S. Pat. No.2,326,059 issued to M. E. Nordberg. A modification of the Nordbergprocess that will provide titanium doped fused silca layer is asfollows. Dry oxygen is bubbled through a tank containing a liquidmixture of approximately 53 percent by weight silicon-tetrachloride,SiCl and 47 percent by weight titanium-tetrachloride, TiCl which mixtureis at a temperature of approximately 35C. SiCl and TiCl vapors picked upby the oxygen are then passed through a gas-oxygen flame where they arehydrolyzed to form a soot, that is, minute glass particles, with acomposition of approximately 95 percent by weight SiO and 5 percent byweight TiO The glass soot leaves the flame in a steady stream, and isdeposited on a rotating starting member. The thickness of the resultingcoating or layer is determined by the amount of soot deposited which isprimarily controlled by the flow rates and the time allowed fordeposition.

ln the formation olTiO containing SiO by flame liydrolysis as heretoforedescribed. it has been found that residual water. that is OH. H and H 0in the glass contribute primarily to the amount of Ti or reduced TiO;present. Three TiO reduction reactions occur that are of particularinterest. These TiO reduction reactions are illustrated as follows:

ZTiO Ti O k 0 2TiO at H Ti O OH ZTiO H Ti O H 0 Reaction l) is a thermalreduction of TiO: to Ti O The equilibrium constant K for reaction l isgiven by the equation |=l 2l 2 a] i 2i where the symbol indicatesconcentration.

Reaction (2) is the reduction of TiO by hydrogen to form OH. Theequilibrium constant K; for reaction (2 is given by the equation 1 i aui I l/l -ZI W21 Hydrogen could also react with TiO to form water and TiO in the glass by reaction (3). The equilibrium constant K for reaction(3) is given by The value of the equilibrium constant for any reactionis related to the free energy change of the reaction by the equation logK AF /4.575 T where AF, is the free energy change of the reaction attemperature T in degrees Kelvin. The free energy changes for resactionsl (2), and 3) have been determined and the resulting free energy versustemperature data are plotted in FIG. 2.

From equation 4, the amount of Ti O in equilibrium with TiO is given byIn FIG. 3, the concentration of Ti in glass is plotted versus glasstemperature for various oxygen concentrations. The effect of reducingthe oxygen concentration is to increase the amount of reduced titania atany temperature, while increasing the temperature also increases theamount of titania reduction.

To obtain the concentration of Ti in glass has has been plotted in FIGS.3, 4, and 5, the following conversion must be made.

The amount of reduced titania produced by the reaction (2) is given by[Ti-10A KziTi zi i ui/ Since the amount of Ti O is dependent on theratio [H [OH], the amount of reduced TiO for various values of [H /[OH]has been determined and these data have been converted to concentrationof Ti" by equation (9) and have been plotted in FlG. 4.

The amount of reduced titania produced by reaction (3) is given by i zai ai zi i 2l i z i ln this case, the amount of Ti O is dependent on theratio [H ]/[H O]. These data have also been converted to concentrationsof Ti as above explained and have been plotted for different values of[Hfl/ [H2O] in FIG. 5. g

The presence of Ti atoms in TiO containing SiO contributes greatly tolight attenuation in the glass. It has been determined that 20 db/kmattenuation can only be achieved if the Ti level in the glass is reducedto 0.l ppb. FIGS. 3, 4, and 5 indicate that in the production of TiO,containing SiO, glass by flame hydrolysis, an appreciable concentrationof Ti" results from reactions l (2), and (3). To illustrate this resultby a specific example, it is assumed that the temperature of the glasssoot particles emitted from the flame hydrolysis burner is I,600C. andthat incomplete combustion of gases in the burner gives values of IO J=1, [H /[OHl= I and [H l/[H O]= 10*. which values are typical forgas-oxygen flames. The amounts of T in the soot glass resulting fromreactions l l, (2), and (3) are determined from FIGS. 3, 4, andS to beabout 10, IO and 10, respectively, at 1,600C. The total Ti level in theglass is therefore seen to be greater than the 0.1 ppb level which mustbe reached to result in no more than about 20 db/km attenuation.

If the temperature of the glass is decreased and the glass is stabilizedat such temperature within an oxygen environment, a decrease in theamount of Ti in the glass can be achieved by the reverse of reaction (b1). Similarly, if the temperature of the glass is decreased and theglass is stabilized at such temperature within an environment, the waterand hydrogen gas content of which is in equilibrium with that of theglass, a decrease in the amount of Ti in the glass can be achieved.These results are illustrated by FIGS. 3, 4, and 5 where, if the glasstemperature is decreased and stabilized at l,200C. under the aboveconditions, the amount of Ti in the glass decreases to about l", l0, andIO, respectively. From the preceding it is seen that reaction (3)involving water is the major cause for obtaining a high level of Ti inthe glass. It has been found that by decreasing the total water andhydrogen gas content in the environment within which the glass ismaintained, a release of residual water and hydrogen gas from the glassis obtained thereby decreasing the Ti content in the glass by thereverse of reactions (2) and (3). This is illustrated in FIG. by dottedlines (I) and (2) which show that by changing the [Hg/[H O] ratio fromto 10 at l,200C., the Ti content in the glass decreases from about 10"to about 10"- when equilibrium between the partial pressure of residualwater and hydrogen gas in the glass and the environment is reached.Similarly, in FIG. 4 it is illustrated by dotted lines (3) and (4) thatin changing the [H /[OI-I] ratio from I to 10' at l,200C., the Ticontent in the glass decreases from about 10 to about 10 whenequilibrium between the partial pressure of hydrogen in the glass andthe environment is reached. Of course, further removal of residual waterand hydrogen gas from the environment will further tend to decrease theTi content in the glass when equilibrium is reached.

Although the above has been described in connection with TiO containingSiO glass, the same applies to other types of glasses which containcompounds of Sb, Sn, U, V, Nb, Mo, Th, Ti, and the like that can existis a reduced state as a result of the same or similar reductionreactions.

Referring now to FIG. 6, there is shown an enclosed chamber 24 withinwhich porous member 22 is disposed. The environment in chamber 24 ismaintained at a degree ofdryness corresponding to pp--.i or less ofwater vapor. This environment may be obtained by connecting pipe 26 to asuitable means for decreasing pressure such as a vacuum means, notshown. The temperature within chamber 24 is maintained at a desiredlevel by heating means or heater 28.

By subjecting porous member 22 to such a controlled environment theresidual water in the glass is removed. In the heretofore describedexample, when residual water is removed from a TiO containing SiO glass,reactions (2) and (3) take place in reverse. That is, in the reverse ofreaction (2), the oxygen from the residual OH combines with the Ti O inthe glass forming TiO and liberating hydrogen which escapes from theporous member into the chamber and subsequently from the chamber throughpipe 26. The reverse of reaction (3) also takes place wherein the oxygenfrom the residual H O molecules oxidizes the Ti O that is presentforming TiO and again liberating hydrogen which escapes from porousmember 22 as heretofore described. In both of these oxidizing reactions,the reduced titanium having a valence of +3 is converted to +4 titaniumthereby reducing the amount of ti present in the member.

Porous member 22 is maintained within the controlled environment forthat period of time which is required for an equilibrium to be reachedbetween the partial pressure of the entrapped gas from the porous memberand the partial pressure of the same gas in the chamber environment. Aswill be hereinafter described in detail, by properly selecting thecontrolled environment within chamber 24, the level of residual waterwhich remains within the member can be reduced to a point wherein itspresence does not significantly effect the utility of the ultimatearticle. The entrapped gas which escapes from porous member 22 isindicated by streams 30 in FIG. 6.

After the residual water is reduced to the desired level, porous member22 is maintained within the controlled environment and heated to atleast the sintering temperature of the glass thereof by means of heater28 to sinter the soot particles that form a consolidated dense member 32as illustrated in FIG. 7. By sintering glass soot particles formed byflame hydrolysis, the resulting consolidated dense member 32 has a sizeapproximately one-half of the size of porous member 22 from which it wasformed.

After consolidated dense member 32 is formed, it may be removed from thecontrolled environment, if desired, for subsequent treatment, such forexample as for removing starting member 12. This is illustrated in FIG.8 wherein starting member 12 is shown being ground out by means of adiamond reamer 34, however, any other means for accomplishing thisresult such, for example, as hydrofluoric acid etching or core drillingare also suitable.

If the consolidated member 32 must be transformed into a different shapethrough any of various glass forming operations, it may be returned tothe predetermined controlled environment for such purposes. If, forexample, an optical waveguide is desired, a cladding over theconsolidated dense member must be formed. The formation of such acladding is illustrated in FIG. 9 wherein a coating 36 of glass havingan index of refraction less than that of the core is applied. If, forexample, the core is formed of Ti0 containing SiO wherein the TiO actsas a dopant to increase the index of refraction of the composition, thecladding may be formed of pure SiO or SiO doped to a lesser degree thanthat of the core. The application of coating 36 may be accomplished bythe flame hydrolysis method heretofore described wherein a flame 16 isemitted from burner 14 and within which flame a gas-vapor mixture ishydrolyzed to form a glass soot as heretofore described. The soot leavesflame 16 in a stream 38 and is directed toward consolidated member 32 asillustrated in FIG. 9. In such an embodiment, it may be desired to alsoremove the residual water from the waveguide cladding, in which case theheretofore described procedure for removing such water is repeated. Onthe other hand, when an optical waveguide is ultimately desired, thecore may be formed as heretofore described and thereafter a coating 36applied thereover before the water is removed and the glass soot issintered. In such a case, the gas-vapor mixture provided to the flamehydrolysis burner is changed to obtain the desired composition of thecladding material after the core is formed. The residual water removaland sintering would not be altered by having two different compositionscomprising the porous member.

ln the optical waveguide embodiment, after both the core and claddingare consolidated as hereinabove described, the resulting structure 40 iseither maintained at temperature or heated by furnace 42 and may bemaintained within the controlled environment as illustrated in FIG. 10,if desired. It may, however, be heated in air at this point of theprocess. After the structure reaches a temperature at which thematerials have a low enough viscosity for drawing, it is drawn untillongitudinal hole 44 collapses, that is the core glass fllls hole 44 toform a solid core. The structure is than further drawn until the crosssectional size thereof is reduced sufficiently to produce fiber 46. Sucha clad fiber thereafter forms the optical waveguide.

A typical example of one embodiment ofthe present invention is asfollows. A starting member of fused quartz, approximately Vs inches indiameter and about l inches long is sealed to a suitable handle. Aliquid mixture containing 30.4 percent by weight TiCl and 69.6 percentby weight SiCl is heated to 35C. Dry oxygen is bubbled through theliquid mixture and SiCl. and TiCl vapors are picked up by the oxygen.This vapor containing oxygen is then passed through a gasoxygen flamewhere the vapors hydrolyze to form a steady stream of approximately 0.1a spherelike particles having a composition of 2.5 percent by weight TiOand 97.5 percent by weight SiO The stream is directed to the startingmember and a soot layer of these particles is applied up to about 1.5inches in diameter. A second coating of lOO percent SiO would than beapplied over the first soot layer by the same flame hydrolysis methoddescribed above except that the liquid mixture does not contain TiCl TheSiO soot would be applied until an outside diameter of approximately 2.5inches is obtained. This structure is then placed in an enclosed chamberwhich is connected to a suitable vacuum producing means. The chamber isevacuated and maintained at less than l0 Torr and is heated incrementlyin steps of about 100C. As the structure increases in temperature, theresidual water is removed by permitting the entrapped gases to escapetherefrom. As the gases escape from the porous member, the predeterminedpressure within the chamber is continually maintained at the same levelby the vacuum means. The structure is heated until an equilibrium isreached between the partial pressure of the entrapped gas within thestructure and the partial pressure of the same gas within the chamber.That is, the structure is heated until the pressure within the chamberno longer increases or tends to increase with an incremental increase intemperature. This equilibrium point is reached for tln: structure of thepresent example in about 24 hours at a temperature of about 1,200C. atwhich time there is less than ppm of water present in the soot.

Thereafter, while maintaining the controlled environment within thechamber, the porous body is heated to at least a temperature of l,400C.,which is the sintering temperature of the glass. to sinter the sootparticles and form a consolidated dense member. After the particles havesintered and a transparent, consolidated. dense glass member isobtained, it is removed from the chamber. The fused quartz startingmember is then ground out by means of a diamond reamer. The tubularmember so formed is rinsed in about a 50 percent hy drofluoric acidsolution, flame polished, and washed again in said acid solution toprovide a clean tubular member. The tubular member is then heated to atemperature of about 2,000C., at which it is drawn. As the structure isdrawn, it decreases in diameter and a central hole collapses. Drawing iscontinued until the final desired optical waveguide dimensions areobtained. The optical waveguide produced in accordance with the aboveexample contains less than 20 ppm of water and has a signal lightattenuation of less than 20 db/km.

A specific example of another embodiment of the present invention is asfollows. All the steps of the above example are the same except that thecontrolled environment within which the porous member was heated was anatmospheric pressure environment having a dew point lower than F. Thisenvironment was achieved by using a liquid gas source and a pressuretight system. The system is flushed out with the gas from said sourceuntil the system contains only such gas. The dew point of such gas wouldcorrespond to the boiling point of the liquid gas. For example, liquidoxygen has a boiling point of 297.4F at one atmosphere pressure. Afteran equilibrium is reached between the partial pressure of the entrappedgas in the structure and the partial pressure ofthe same gas in the saidenvironment, the porous member is heated to at least the sinteringtemperature of the glass soot and the soot particles are sintered toform a consolidated dense member within such environment having a dewpoint of less than 130F. A waveguide produced by the method of thisembodiment would also result in one having less than 20 ppm of water anda light signal attenuation of less than 20 db/km.

Although the present invention has been described with respect tospecific details of certain embodiments thereof, it is not intended thatsuch details be limitations upon the scope of the invention exceptinsofar as set forth in the following claims.

I claim:

1. The method of forming an article comprising the steps of providing asuitable starting member,

forming glass soot by flame hydrolysis,

depositing a quantity of said glass soot on said starting member to forma porous body thereof,

disposing said porous body in a controlled environment,

maintaining a predetermined desired concentration of gases in saidenvironment,

heating said porous body to a temperature below the sinteringtemperature of said glass to permit entrapped gas to escape therefrom,

maintaining said temperature until an equilibrium is reached between thepartial pressure of the entrapped gas in said body and the partialpressure of the same gas in said environment, thereafter heating saidporous body to at least the sintering temperature of said glass tosinter said soot particles and form a consolidated dense member,

forming said member to a desired shape, and

removing the article so formed from said environment.

2. The method of claim 1 further comprising the step of removing saidstarting member before the step of forming said member to a desiredshape.

3. The method of claim I wherein said controlled environment containsless than about 20 parts per million of water vapor.

4. The method of claim 1 wherein said glass soot is fused silica dopedwith at least one material selected from the group consisting oftitanium oxide, tantalum oxide, tin oxide, niobium oxide, zirconiumoxide, aluminum oxide, lanthanum oxide, and germanium oxide.

5. The method of claim 4 wherein said soot is fused silica doped withnot more than percent by weight titanium oxide.

6. The method of claim 5 wherein said article so formed contains lessthan parts per million of residual water.

7. The method of claim 5 wherein said porous body is heated to atemperature of about l,200C.

8. The method of claim 7 wherein said temperature is maintained for aperiod of at least 24 hours.

9. The method of claim 1 further comprising the steps of applying acoating of glass soot having an index of refraction less than that ofthe glass of said porous body over the outside peripheral surface ofsaid porous body before it is disposed within said controlledenvironment.

10. The method of claim 9 wherein said controlled environment containsless than about 20 parts per million of water vapor.

11. The method of claim 9 wherein said coating of glass soot is appliedby flame hydrolysis.

12. The method of claim 11 wherein said porous body is formed of dopedfused silica and said coating is formed of fused silica doped to alesser degree than that of said porous body.

13. The method of claim 12 wherein said porous body is fused silicadoped with at least one material selected from the group consisting oftitanium oxide, tantalum oxide, tin oxide, niobium oxide, zirconiumoxide, aluminum oxide, lanthanum oxide, and germanium oxide.

14. The method of claim 13 wherein said porous body is formed of fusedsilica doped with not more than 15 percent by weight of titanium oxide.

15. The method of claim 14 wherein said porous body and said coating aresintered simultaneously.

16. The method of claim 15 wherein said article so formed contains lessthan 20 parts per million of residual water.

17. The method of claim 1 further comprising the step of applying acoating a glass soot over the consolidated dense member.

18. The method of claim 17 further comprising the steps of disposing thestructure so formed in a controlled environment,

maintaining a predetermined desired concentration of gases in saidenvironment, heating said structure to a temperature below the sinteringtemperature of the glass thereof to permit entrapped gas to escape fromsaid porous coating,

maintaing said temperature until an equilibrium is reached between thepartial pressure of the entrapped gas within said porous coating and thepartial pressure of the same gas in said environment, and

heating said structure to at least the sintering temperature of theglass of said soot to sinter said soot particles and form a consolidateddense coating over said consolidated member.

19. The method of claim 18 wherein said controlled environment containsless than about 20 parts per million of water vapor.

20. The method of claim 18 further comprising the step of removing saidstarting member.

21. The method of claim 20 wherein said forming said member comprisesthe steps of heating the structure so formed to the drawing temperatureof the materials thereof, and

drawing the heated structure to reduce the cross sectional area thereofand to collapse the hole resulting from removing said starting memberand to form a clad fiber having a solid cross-section, the collapsedconsolidated dense member forming the core of said fiber and saidcoating forming the fiber cladding.

22. The method of claim 21 wherein said drawing is performed within saidenvironment.

23. The method of claim 22 wherein said article so formed contains lessthan 20 parts per million of residual water.

24. An article formed by the method of claim 1. =l

1. THE METHOD OF FORMING AN ARTICLE COMPRISING THE STEPS OF PROVIDING ASUITABLE STARTING MEMBER, FORMING GLASS SOOT BY FLAME HYDROLYSIS,DEPOSITING A QUANTITY OF SAID GLASS SOOT ON SAID STARTING MEMBER TO FORMA POROUS BODY THEREOF, DISPOSING SAID POROUS BODY IN A CONTROLLEDENVIRONMENT, MAINTAINING A PREDETERMINED DESIRED CONCENTRATION OF GASESIN SAID ENVIRONMENT, HEATING SAID POROUS BODY TO A TEMPERATURE BELOW THESINTERING TEMPERATURE OF SAID GLASS TO PERMIT ENTRAPPED GAS TO ESCAPETHEREFROM, MAINTAINING SAID TEMPERATURE UNTIL AN EQUILIBRIUM IS REACHEDBETWEEN THE PARTIAL PRESSURE OF THE ENTRAPPED GAS IN SAID BODY AND THEPARTIAL PRESSURE OF THE SAME GAS IN SAID ENVIRONMENT, THEREAFTER HEATINGSAID POROUS BODY TO AT LEAST THE SINTERING TEMPERATURE OF SAID GLASS TOSINTER SAID SOOT PARTICLES AND FORM A CONSOLIDATED DENSE MEMBER, FORMINGSAID MEMBER TO A DESIRED SHAPE, AND REMOVING THE ARTICLE SO FORMED FROMSAID ENVIRONMENT.
 2. The method of claim 1 further comprising the stepof removing said starting member before the step of forming said memberto a desired shape.
 3. The method of claim 1 wherein said controlledenvironment contains less than about 20 parts per million of watervapor.
 4. The method of claim 1 wherein said glass soot is fused silicadoped with at least one material selected from the group consisting oftitanium oxide, tantalum oxide, tin oxide, niobium oxide, zirconiumoxide, aluminum oxide, lanthanum oxide, and germanium oxide.
 5. Themethod of claim 4 wherein said soot is fused silica doped with not morethan 15 percent by weight titanium oxide.
 6. The method of claim 5wherein said article so formed contains less than 20 parts per millionof residual water.
 7. The method of claim 5 wherein said porous body isheated to a temperature of about 1,200*C.
 8. The method of claim 7wherein said temperature is maintained for a period of at least 24hours.
 9. The method of claim 1 further comprising the steps of applyinga coating of glass soot having an index of refraction less than that ofthe glass of said porous body over the outside peripheral surface ofsaid porous body before it is disposed within said controlledenvironment.
 10. The method of claim 9 wherein said controlledenvironment contains less than about 20 parts per million of watervapor.
 11. The method of claim 9 wherein said coating of glass soot isapplied by flame hydrolysis.
 12. The method of claim 11 wherein saidporous body is formed of doped fused silica and said coating is formedof fused silica doped to a lesser degree than that of said porous body.13. The method of claim 12 wherein said porous body is fused silicadoped with at least one material selected from the group consisting oftitanium oxide, tantalum oxide, tin oxide, niobium oxide, zirconiumoxide, aluminum oxide, lanthanum oxide, and germanium oxide.
 14. Themethod of claim 13 wherein said porous body is formed of fused silicadoped with not more than 15 percent by weight of titanium oxide.
 15. Themethod of claim 14 wherein said porous body and said coating aresintered simultaneously.
 16. The method of claim 15 wherein said articleso formed contains less than 20 parts per million of residual water. 17.The method of claim 1 further comprising the step of applying a coatinga glass soot over the consolidated dense member.
 18. The method of claim17 further comprising the steps of disposing the structure so formed ina controlled environment, maintaining a predetermined desiredconcentration of gases in said environment, heating Said structure to atemperature below the sintering temperature of the glass thereof topermit entrapped gas to escape from said porous coating, maintaing saidtemperature until an equilibrium is reached between the partial pressureof the entrapped gas within said porous coating and the partial pressureof the same gas in said environment, and heating said structure to atleast the sintering temperature of the glass of said soot to sinter saidsoot particles and form a consolidated dense coating over saidconsolidated member.
 19. The method of claim 18 wherein said controlledenvironment contains less than about 20 parts per million of watervapor.
 20. The method of claim 18 further comprising the step ofremoving said starting member.
 21. The method of claim 20 wherein saidforming said member comprises the steps of heating the structure soformed to the drawing temperature of the materials thereof, and drawingthe heated structure to reduce the cross sectional area thereof and tocollapse the hole resulting from removing said starting member and toform a clad fiber having a solid cross-section, the collapsedconsolidated dense member forming the core of said fiber and saidcoating forming the fiber cladding.
 22. The method of claim 21 whereinsaid drawing is performed within said environment.
 23. The method ofclaim 22 wherein said article so formed contains less than 20 parts permillion of residual water.
 24. An article formed by the method of claim1.